Suitability of AuRM2 as a Reference Material for Trace Element Microanalysis of Native Gold

Microanalysis of native gold specimens has been hampered by the lack of a suitable reference material (RM) known to be sufficiently homogeneous at the scale of microanalytical sampling. The suitability of gold reference material AuRM2 for microanalysis was assessed. This RM was created for bulk analysis of refined gold and was only certified for homogeneity at the bulk scale. However, it contains trace elements in appropriate mass fraction ranges for analysis of native gold. This study was not intended to provide alternative mass fractions from the original certified values, only to assess its suitability for microanalytical methods. Micro-scale (similar to 3.4g sample mass) heterogeneity was calculated from measurement repeatability of LA-ICP-MS analyses of AuRM2 by factoring in signal (represented by counting statistics) and instrument set-up-specific variability (determined using measurement variability of a reference material known to be homogeneous). Elements determined to be homogeneous or to have minor heterogeneity (<10% calculated heterogeneity RSD) are Mg, Al, Ti, Fe, Ni, Cu, Zn, Se, Rh, Sn, Sb, Pt and Pb. Elements with moderate heterogeneity (10-20% heterogeneity RSD) are: Mn, As, Pd, Te and Bi. Correlation of element mass fractions indicates that micro-scale inclusions of chalcophile-rich phases along grain boundaries may be responsible for some of the chemical heterogeneity. However, the level of heterogeneity is statistically negligible compared with the ranges of chemical signatures observed in sample populations of native gold. Therefore, AuRM2 is shown to be sufficiently homogenous at a micro-scale for use as a RM for microanalysis of native gold.